1. Field of the Invention
The present invention relates generally to production of electric arc discharge lamps and, more particularly, to an annular dosing capsule for use in an electric discharge lamp and a method employing the capsule for dosing the lamp with the desired quantity of a dosing material, such as liquid mercury and like substances.
2. Description of the Prior Art
In the manufacture of electric discharge lamps, such as fluorescent lamps, which utilize an ionizable medium containing mercury, it is necessary to introduce a quantity of the mercury into a sealed envelope of the lamp where the mercury will be employed as a vapor in the production of light. To place the desired quantity of mercury into the fluorescent lamp, one approach has been to employ a mercury dosing apparatus. One conventional dosing apparatus utilized heretofore is operable to, first, form a droplet of liquid mercury external to the lamp and, then, blow the liquid mercury droplet into the lamp with a flush or fill gas at a stage in the lamp production process prior to hermetically closing and sealing the lamp envelope.
Due to inaccuracies of metering and losses of mercury during transport from the respective apparatus into the lamp, such prior art mercury dosing apparatus has not been found capable of dispensing a precisely or accurately measured quantity of mercury, preferably in the form of a single piece or ball of mercury, into lamps on a repeatable basis. To compensate for this deficiency, in most instances a substantially larger quantity of mercury than is actually needed for operation of the lamp is intentionally introduced to ensure that, at least, the minimum quantity of mercury will be present in the lamp envelope to provide adequate lamp performance and useful life.
Because of the adverse effects of mercury on the environment it would be highly desirable to be able to avoid the overuse of mercury in the manufacture of gas discharge lamps. Because a minimum quantity of mercury is needed in a lamp to meet design life requirements, reducing mercury in the lamp requires reducing the variability of the dosing technique.
An alternative approach to providing accurate dosing has been to sealably encapsulate the desired quantity of mercury in a heat resistive media, usually glass or metal, and attach the mercury capsule to one of the electrode mounts so that the capsule will be located inside of the lamp envelope after sealing the lamp. U.S. Pat. Nos. 4,494,042, 4,553,067 and 4,823,047 disclose this alternative approach. The purpose for using the mercury capsule is to allow the mercury to stay intact and isolated from the rest of the lamp atmosphere until after the remaining operations of sealing, exhausting and tipping of the lamp are complete. Then, by the application of intense induction heating, the capsule can be ruptured, allowing the escape of mercury into the lamp atmosphere.
In one current dosing method employing a mercury capsule, a shield has been used for attaching the mercury capsule to the electrode mount. The use of the shield has been required to provide the continuous current path needed for induction heating. However, the shield adds cost to the lamp and limits the range of potential lamp designs. Also, the use of the shield with induction heating requires the use of high cost and maintenance equipment both on the mount making machines and on equipment used subsequent to the exhaust process. The induction heating takes time, and in a high speed manufacturing operation, time translates into increase machine length and increased equipment and facility investment.
Therefore, a need remains for another approach to mercury dosing of electric discharge lamps which eliminates the problems associated with prior art mercury dosing approaches without substituting other problems in their place.